Health monitoring of an electrochemical cell stack

ABSTRACT

The present disclosure is directed towards a method and a system for monitoring the performance of an electrochemical cell stack. Monitoring can be performed remotely by measuring the voltage across the stack, and comparing the measured values to predetermined reference values to determine the condition of the stack. Monitoring of the stack voltage enables detection of performance decay, which in turn enables preemptive repair of the stack prior to catastrophic failure.

This application is a division of U.S. patent application Ser. No.14/296,974, filed Jun. 5, 2014, and claims the benefit of U.S.Provisional Application No. 61/832,378, filed Jun. 7, 2013, all of whichis incorporated herein by reference.

The present disclosure is directed towards electrochemical cells, andmore specifically, a method and a system for monitoring anelectrochemical cell or a stack of cells.

Electrochemical cells, usually classified as fuel cells or electrolysiscells, are devices used for generating current from chemical reactions,or inducing a chemical reaction using a flow of current. A fuel cellconverts the chemical energy of a fuel (e.g., hydrogen, natural gas,methanol, gasoline, etc.) and an oxidant (air or oxygen) intoelectricity and waste products of heat and water. A basic fuel cellcomprises a negatively charged anode, a positively charged cathode, andan ion-conducting material called an electrolyte.

Different fuel cell technologies utilize different electrolytematerials. A Proton Exchange Membrane (PEM) fuel cell, for example,utilizes a polymeric ion-conducting membrane as the electrolyte. In ahydrogen PEM fuel cell, hydrogen atoms are electrochemically split intoelectrons and protons (hydrogen ions) at the anode. The electrons flowthrough the circuit to the cathode and generate electricity, while theprotons diffuse through the electrolyte membrane to the cathode. At thecathode, hydrogen protons combine with electrons and oxygen (supplied tothe cathode) to produce water and heat.

An electrolysis cell represents a fuel cell operated in reverse. A basicelectrolysis cell functions as a hydrogen generator by decomposing waterinto hydrogen and oxygen gases when an external electric potential isapplied. The basic technology of a hydrogen fuel cell or an electrolysiscell can be applied to electrochemical hydrogen manipulation, such as,electrochemical hydrogen compression, purification, or expansion.Electrochemical hydrogen manipulation has emerged as a viablealternative to the mechanical systems traditionally used for hydrogenmanagement. Successful commercialization of hydrogen as an energycarrier and the long-term sustainability of a “hydrogen economy” largelydepend on the efficiency and cost-effectiveness of fuel cells,electrolysis cells, and other hydrogen manipulation/management systems.

In operation, a single fuel cell can generally generate about 1 volt. Toobtain the desired amount of electrical power, individual fuel cells arecombined to form a fuel cell stack. The fuel cells are stacked togethersequentially, each cell including a cathode, an electrolyte membrane,and an anode. Each cathode/membrane/anode assembly constitutes a“membrane electrode assembly”, or “MEA”, which is typically supported onboth sides by bipolar plates. Gases (hydrogen and air) are supplied tothe electrodes of the MEA through channels or grooves formed in theplates, which are known as flow fields. In addition to providingmechanical support, the bipolar plates (also known as flow field platesor separator plates) physically separate individual cells in a stackwhile electrically connecting them in series.

Additionally, a typical fuel cell stack includes manifolds and inletports for directing the fuel and oxidant to the anode and cathode flowfields, respectively. The stack may also include a manifold and inletport for directing a coolant fluid to interior channels within the stackto absorb heat generated during operation of the individual cells. Afuel cell stack also includes exhaust manifolds and outlet ports forexpelling the unreacted gases and the coolant water.

FIG. 1 is an exploded schematic view showing the various components of aPEM fuel cell 10. As illustrated, bipolar plates 2 flank the “membraneelectrode assembly” (MEA), which comprises an anode 7A, a cathode 7C,and an electrolyte membrane 8. Hydrogen atoms supplied to anode 7A areelectrochemically split into electrons and protons (hydrogen ions). Theelectrons flow through an electric circuit to cathode 7C and generateelectricity in the process, while the protons move through electrolytemembrane 8 to cathode 7C. At the cathode, protons combine with electronsand oxygen (supplied to the cathode) to produce water and heat.

Additionally, PEM fuel cell 10 comprises electrically-conductive gasdiffusion layers (GDLs) 5 within the cell on each side of the MEA. GDLs5 serve as diffusion media enabling the transport of gases and liquidswithin the cell, provide part of the electrical conduction path betweenbipolar plates 2 and electrolyte membrane 8 through the flow fields 4,6. GDLs 5 can also aid in the removal of heat and process water from thecell, and in some cases, provide mechanical support to electrolytemembrane 8. GDLs 5 can comprise a woven or non-woven carbon or otherconductive material cloth with electrodes 7A and 7C located on the sidesfacing the electrolyte membrane. In some cases, the electrodes 7A and 7Cinclude an electrocatalyst material coated onto either the adjacent GDL5 or the electrolyte membrane 8. Some high pressure or high differentialpressure fuel cells use “frit”-type densely sintered metals, screenpacks, expanded metals, metal foam, or three-dimensional porous metallicsubstrates in combination with or as a replacement for traditional GDLsto provide structural support to the MEA in combination withtraditional, land-channel flow fields 4, 6 formed in the bipolar plates2. In a typical fuel cell, reactant gases on each side of theelectrolyte membrane flow through the flow fields and then diffusethrough the porous GDL to reach the electrolyte membrane.

The performance of an electrochemical cell or a cell stack can decayover time for a number of reasons, including, but not limited to,flooding of the cells with water, contamination of the reactant gases,fuel crossover, back diffusion, poisoning of the membranes or catalysts,etc. Fuel crossover in particular, i.e., leakage of fuel from the anodeside to the cathode side, is a cause for serious concern regarding thesafety and efficiency of an electrochemical cell. Fuel crossover canoccur around or through the electrolyte membrane of the MEA, and isgenerally caused by either leaky seals between the cell components,puncture or tearing of the membrane, or naturally occurring seepagethrough the membrane. Fuel leakage across the membrane is highlyundesirable because it can cause direct combustion reaction betweenoxygen (supplied to the cathode) and the fuel and damage the cell/stack.Further, crossover results in fuel wastage; the fuel can possibly leakinto the coolant supply and cause further contamination; or the fuel cancause external leaks and contaminate the entire cell stack environment.

Fuel crossover, back diffusion, flooding, contamination, and variousother factors that influence the performance of a cell stack, cause adrop in voltage across the cell/stack and loss of efficiency. Forinstance, in an electrochemical hydrogen compression (EHC) cell stack,the hydrogen output is directly proportional to the current through thestack; increase in the required voltage to deliver a given current is adirect indication of cell/stack degradation. Therefore, when any ofthese damaging conditions arise, corrective action is necessary toprevent irreversible cell/stack degradation and catastrophicin-the-field failures.

The present disclosure is directed towards a method and a system formonitoring the performance of a cell or a stack of cells to preemptivelyidentify damaging conditions resulting in performance decay so as toenable repair/service of the cell/stack prior to failure. The monitoringcan be performed remotely by measuring the voltage across the cell/stack(referred to hereinafter as the “stack voltage”) and comparing themeasured values to predetermined reference values to determine thecondition of a cell/stack.

A first aspect of the present disclosure is an electrochemical cellsystem comprising an electrochemical cell, a voltage measurement deviceadapted to measure a voltage across the cell, and a control unitconfigured to process voltage signals received from the voltagemeasurement device, wherein the control unit is configured to conduct alinear current sweep across the cell.

Another aspect of the present disclosure is a method for monitoring anelectrochemical cell. The method comprises the steps of conducting alinear current sweep across the cell, generating a baselinevoltage-current (V-I) curve of the cell and calculating a slope of thebaseline V-I curve, generating an aged V-I curve of the cell at any timepoint during operation of the cell and calculating a slope of the agedV-I curve, calculating a difference in value between the slopes of thebaseline V-I curve and the aged V-I curve, and comparing the differencein value between the slopes of the baseline V-I curve and the aged V-Icurve to a preset reference value.

Yet another aspect of the present disclosure is a method for monitoringan electrochemical cell. The method comprises the steps of measuring avoltage required to deliver a given current through the cell at thebeginning of the cell's life to establish a baseline voltage, measuringa voltage required to deliver a given current through the cell at anytime point during operation of the cell to establish an aged voltage,calculating a difference between the baseline voltage and the agedvoltage, and comparing the difference in value between the baselinevoltage and the aged voltage to a preset reference value

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description, serve to explain the principles of thevarious aspects of the invention.

FIG. 1 illustrates an exploded schematic view showing the variouscomponents of a Proton Exchange Membrane (PEM) fuel cell;

FIG. 2A illustrates a schematic view showing voltage measurement acrossan exemplary cell stack;

FIG. 2B illustrates a V-I curve generated using a linear current sweepconducted at the beginning of life (BOL) of an electrochemical cellstack, in accordance with exemplary embodiments of the presentdisclosure; and

FIG. 2C illustrates a V-I curve generated using a linear current sweepconducted after field operation of the electrochemical cell stackidentified in FIG. 2B, in accordance with exemplary embodiments of thepresent disclosure.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

Reference will now be made to certain embodiments consistent with thepresent disclosure, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused throughout the drawings to refer to the same or like parts. It isto be understood that the systems and methods of the present disclosurecan be employed with various types of electrochemical cells, including,but not limited to, high pressure and low pressure cells, highdifferential pressure cells (e.g., electrochemical hydrogen compressor),cells with a low rate of heat generation, as well as cells operating ata high rate of heat generation. Also, it is to be understood thatalthough the present disclosure is described in relation to anelectrochemical cell stack, the systems and methods of the presentdisclosure can be employed with individual electrochemical cells aswell.

Electrochemical cell stacks comprise a plurality of individual cells. Inan exemplary cell stack 100, shown in FIG. 2A, each cell comprises a“membrane electrode assembly” (MEA), which includes an anode, a cathode,and an electrolyte membrane. In exemplary embodiments, the cellcomprises a GDL and bipolar plate on each side of the MEA to direct thereactant gases through the cell. In some embodiments, eachelectrochemical cell in a cell stack comprises two bipolar plates, oneon each side of the membrane-electrode-assembly (MEA) i.e., if the stackcomprises n cells, then the total number of bipolar plates in the stackis 2n. In some other embodiments, two adjacent electrochemical cells ina stack share a bipolar plate, i.e., if the stack comprises n cells,then the total number of bipolar plates in the stack is (n+1). In suchembodiments, a single bipolar plate can have flow field features on bothsides of the plate—for instance, one side of the plate supports the GDLof one cell and the other side supports the GDL of an adjoining cell.

In exemplary embodiments, the cell stack further comprises pressuretransmitters arranged in the reactant fluid inlet and outlet. Thepressure transmitters can be configured to measure the pressure of thereactant fluid at the inlet (“P_(in)”) and the pressure at the outlet(“P_(out)”), and to report the measured pressure values to a controlsystem associated with the electrochemical cell stack. The cell stackcan further include one or more voltage monitoring devices 105, e.g.,voltmeters, to measure the total voltage across the stack. In selectembodiments, the cell stack may comprise additional voltage measuringdevices to measure the voltage across individual cells. Further, in someembodiments, the cell stack may comprise one or more ammeters 110 tomeasure the current through the cells.

In exemplary embodiments, the voltage across the cell stack(“V_(stack)”) and current through the cell “I_(stack)” are measured inreal-time, as shown in FIG. 2A. The measured signal is transmitted tothe control system, which may be a remote unit. In select embodiments,the signal can be transmitted wirelessly to the remote control systemassociated with the stack. The control system is configured to generatea linear current sweep at the beginning of life (“BOL”) of the cellstack to establish a baseline V-I characteristic of the stack, asindicated in FIG. 2B. The baseline V-I curve is established using apredetermined ratio of P_(in) and P_(out). The control system calculatesslope of the V-I curve, which provides the effective resistance(“R_(eff)”) of the stack. “R_(eff)” is the average resistivity of thecells in the stack, measured in ohms-cm². “R_(eff)” may be calculated bysubtracting the Nernst voltage of a cell from the actual voltage acrossthe cell. In select embodiments, R_(eff) is measured according toequation (1) below:

$\begin{matrix}{R_{eff} = \frac{\frac{V_{stack}}{N_{cell}} - {\frac{RT}{2F}{\ln \left( \frac{P_{out}}{P_{in}} \right)}}}{\frac{I_{stack}}{{AA}_{cell}}}} & (1)\end{matrix}$

wherein “N_(cell)” is the number of cells in the stack; “T” is theoperating temperature of the stack; “AA” is the active area of the cell(ranging from about 10 cm² to about 2000 cm²; typically about 250 cm²);“F” is Faraday's constant (96485.3 Coulombs/mol); “T” is the averageabsolute temperature of the stack (provided in kelvin (K) unit ofmeasurement); “R” is the universal gas constant (8.31451 J/mol K).

The current sweep can be repeated periodically, and the slope(“R_(eff)”) of the stack can be calculated for each repetition of thecurrent sweep. The value of the slope at a particular time point can becompared to the value of the slope at BOL, and the difference betweenthe two values (“ΔSlope”) can be recorded. As the stack ages, the slopeof the V-I curve can increase, which is a direct indication ofperformance degradation of the stack due to various reasons, such as,fuel crossover, impurities in the incoming fuel, etc. Increase in theslope of the V-I curve in effect increases the Slope value.

In select embodiments, the control system is configured to measure thevoltage at a given current value, instead of measuring the V-I slope.Rise in voltage across the stack (“ΔV”) indicates that the amount ofvoltage required to deliver a given current increases, which in turnpoints to performance degradation of the stack. The Slope (or ΔV) valuefor each measurement can then be compared against a predeterminedreference value. If the Slope (or ΔV) is greater than the referencevalue, then corrective measures can be taken, for example, an alarm canbe triggered to alert the operator of the stack of potential undesirableconditions. In some embodiments, the control system can be configured toautomatically initiate preventative actions, for example, triggeringshutdown of the stack, initiating a purge cycle to automatically cleanthe incoming fuel line, etc. In select embodiments, the reference valuecan be varied over time to accommodate changes in Slope (or ΔV) valuesdue to normal cell degradation over time.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A method of monitoring an electrochemical cell,comprising: conducting a linear current sweep across the cell; measuringthe pressure at an inlet (P_(in)) and at an outlet (P_(out)) of thecell; generating a baseline voltage-current (V-I) curve of the cell andcalculating a slope of the baseline V-I curve; generating an aged V-Icurve of the cell at any time point during operation of the cell using apredetermined ratio of pressures of P_(in) and P_(out) and calculating aslope of the aged V-I curve; calculating a difference in value betweenthe slopes of the baseline V-I curve and the aged V-I curve; andcomparing the difference in value between the slopes of the baseline V-Icurve and the aged V-I curve to a preset reference value.
 2. The methodof claim 1, further comprising alerting an operator of the cell if thedifference in value between the slopes of the baseline V-I curve and theaged V-I curve is greater than the preset reference value.
 3. The methodof claim 1, further comprising initiating automatic corrective actionsif the difference in value between the slopes of the baseline V-I curveand the aged V-I curve is greater than the preset reference value. 4.The method of claim 1, further comprising shutting down theelectrochemical cell if the difference in value between the slopes ofthe baseline V-I curve and the aged V-I curve is greater than the presetreference value.
 5. A method of monitoring an electrochemical cell,comprising: measuring a voltage required to deliver a given currentthrough the cell at the beginning of the cell's life to establish abaseline voltage; measuring the pressure at an inlet (P_(in)) and at anoutlet (P_(out)) of the cell; measuring a voltage required to deliver agiven current through the cell at any time point during operation of thecell to establish an aged voltage; calculating a difference between thebaseline voltage and the aged voltage at a predetermined ratio ofpressures of P_(in) and P_(out); and comparing the difference in valuebetween the baseline voltage and the aged voltage to a preset referencevalue.
 6. The method of claim 5, further comprising alerting an operatorof the cell if the difference in value between the baseline voltage andthe aged voltage is greater than the preset reference value.
 7. Themethod of claim 5, further comprising initiating automatic correctiveactions if the difference in value between the baseline voltage and theaged voltage is greater than the preset reference value.
 8. The methodof claim 5, further comprising shutting down the electrochemical cell ifthe difference in value between the baseline voltage and the agedvoltage is greater than the preset reference value.